Pneumatic actuator system

ABSTRACT

A pneumatic actuator system is provided which includes one or more piston-cylinder type actuators ( 14 ) intended for crust breaking operations at electrolytic alumina reduction baths. Each actuator ( 14 ) includes a working piston ( 21 ), and a piston rod ( 22 ) carrying a crust breaking working implement ( 17 ). A control circuit having a directional valve ( 24 ) is arranged to operate the actuator piston ( 21 ) in alternative directions. The control circuit includes air feed flow restrictions ( 26, 27 ), end position sensors ( 28, 29 ) and air feed shut-off valves ( 30, 31 ) for minimizing the pressure air volume needed for accomplishing complete working strokes of the actuator piston ( 21 ) at varying crust layer thickness.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/SE01/01729 filed Aug. 10, 2001.

This invention relates to a pneumatic actuator system including one ormore piston-cylinder type actuators, each having a working piston with aload engaging piston rod. The system further comprises a control circuitwith a directional valve for directing pressure air to alternative sidesof the working piston of each actuator for accomplishing movement of theworking piston in alternative directions, and flow restrictions forrestricting the air feed flow to the actual driving side of the workingpiston.

Actuator systems of this kind are used in the aluminium producingindustry, in particular for crust breaking operations in electrolyticalumina reduction pots. Aluminium producing plants are usually bigoperations having a great number of electrolytic baths for reduction ofaluminium oxide into metallic aluminium. For repeatedly breaking thecrust layers inevitably formed on top of the electrolytic baths andthereby enabling supply of alumina, i.e. pulverized aluminium oxide intothe baths, there are used a great number of big-size pneumaticactuators.

A problem inherent in this type of operations is that the crust layersto be broken may vary in thickness from zero to a very massive crustbody, and to be able to deal with the thicker crust layers the actuatorshave to be big and powerful. For a big aluminium producing plant thiscreates a demand for a huge pressure air supply capacity, becausedriving the working piston of each actuator in reciprocating cyclesrequires a large amount of pressure air. This causes substantial costs,and there is a serious need in this type of industry to reduce theoverall pressure air consumption and to bring down these costs

Previously, a solution to this problem has been suggested which meansthat the current driving side of the actuator working piston is fed withpressure air via a flow restriction, whereas the opposite idling side ofthe working piston is vented through a substantially unrestrictedoutlet. This means that the pressure on the driving side of the workingpiston is quite low as long as the resistance to the piston movement islow, but increases automatically all the way up to the maximum pressureavailable in case the resistance to piston movement becomes higher.

In the above described field of use for pneumatic actuators, the crustlayers are very thin and result in very low piston loads in more than90% of all crust breaking cycles. In less than 1% of all cycles, thecrusts are thick enough to require a full power action. This means thatin a vast majority of the crust breaking cycles, the required airpressure behind the working piston is very low, as is the pressure airvolume fed into the actuator cylinder. The above described restrictedair feed to the actuator means a certain reduction in the consumedpressure air volume compared to previously used full pressure actuatoroperations, and of course it means a substantial cost saving for theindustry. A condition for this, however, is that the piston is allowedto return to its start position immediately after reaching its extendedextreme position, otherwise, there will still be a full pressurebuild-up in the actuator cylinder and a resulting pressure air waste.

Due to reasons as customer requirements and slow signal communicationbetween position sensing means at the electrolytic pot and a controlunit, the piston in previous actuators has been maintained for some timein its extended end position, which means that even if you use feed flowrestrictions to keep down the drive pressure on the piston during pistonmovement, there will still be a full pressure build-up in the actuatorcylinder after the piston has completed its strokes. Such pressurebuild-ups are of no use but a waste of expensive pressure air.

The main object of the present invention is to accomplish a pneumaticactuator system by which the pressure air consumption is brought down toa minimum such that no more pressure air than absolutely necessary isspent on the actuator operation while automatically providing maximumpressure and top power capacity when ever required.

Another object of the invention is to provide a pneumatic actuatorsystem having short and quick air communication routes, so as to makethe actuator operation distinct and without any delays in relation togiven command signals.

A further object of the invention is to enable operation of more thanone actuator by a single directional valve.

A still further object of the invention is to provide an actuator systemwherein components sensitive to harsh environmental factors like heat,strong magnetic fields, chemically active substances etc. may be locatedremotely from the actuator without increasing the pressure airconsumption.

Other objects and advantages of the invention will appear from thefollowing specification containing a detailed description of preferredembodiments of the invention with reference to the accompanyingdrawings.

In the drawings:

FIG. 1 illustrates schematically a section through an electrolytic bathin an aluminium producing plant, including a pneumatic actuator forcrust breaking purposes.

FIG. 2 shows schematically an actuator system according to oneembodiment of the invention.

FIG. 3 shows an actuator system according to an alternative embodimentof the invention.

FIG. 4 shows an actuator system according to a second alternativeembodiment of the invention.

As mentioned above, the pneumatic actuator system according to theinvention is suitable for crust breaking operations in the aluminiumproducing industry. One type of aluminium producing plant comprises anumber of electrolytic pots, and in FIG. 1 there is shown one suchelectrolytic pot 10 containing an electrolytic bath 11 and having abottom cathode 12 and two anodes 13. The anodes 13 are movably supportedon an overhead structure 15 (not shown in detail), and a singlepneumatic actuator 14 mounted on the same structure 15. On top of theelectrolyte 11, there is inevitably formed a crust layer 16 comprisingresidual material from the alumina reduction process.

As an electrolytic reduction process is going on, a crust layer iscontinuously formed on top of the bath, and to be able to add morealumina to the bath during the process the crust layer has to berepeatedly broken. To this end, the pneumatic actuator 14 is mountedvertically and provided with a crust breaking working implement 17, andwhen it is decided to accomplish a hole in the crust layer 16, theactuator 14 is activated to force the working implement 17 right throughthe crust layer. For adding alumina to the bath there is provided a socalled point feeding device by which alumina is supplied right throughthe hole made by the working implement 17. The alumina feeding device isnot a part of the invention and is therefore not described in furtherdetail.

In FIG. 2 there is described an actuator system according one embodimentof the invention which comprises a piston-cylinder type actuator 14having a cylinder 20, a piston 21 and a piston rod 22. The latter isintended to engage an external load of varying magnitude, for instancevia a crust breaking implement 17 as described above. The system furthercomprises an actuator control circuit which includes a directional valve24 connected to a pressure air source 25 and which has air communicationports for directing pressure air to and from the actuator 14. Thedirectional valve 24 is spring biassed in one direction and pressure airactivated by a start command signal in the opposite direction. The startcommand signal is supplied via a conduit 23. Alternatively, the startcommand signal may be provided as an electrical signal from a remotecontrol unit for actuating an electromagnetic air valve located close tothe directional valve 24.

The directional valve 24 shown in FIG. 2 also comprises flowrestrictions 26,27 located in the alternative air feed passages throughwhich pressure air is supplied to the actuator 14. Alternatively, theseflow restrictions may be replaced by a single restriction located at theinlet port of the directional valve 24. However, the purpose andfunctional features of the flow restrictions 26,27 will appear from thefollowing specification.

The control circuit further comprises two end position sensing valves28,29 which are built-in in the actuator cylinder 20 for detecting andindicating whether the piston 21 has reached its extreme end positions.

Two air shut-off valves 30,31 are provided to alternatively let throughor block air flow to and from the actuator 14, respectively, dependenton the current position of the piston 21 as detected by the end positionsensing valves 28,29. Whereas the position sensing valves 28,29 aremechanically activated by the piston 21, the air shut-off valves 30,31are pressure air activated. The position sensing valves 28,29 are springbiassed towards their closed positions, whereas the air shut-off valves30,31 are spring biassed towards their open positions.

In operation of the actuator system, the directional valve 24 is given astart command signal via the conduit 23, whereby the valve 24 is shiftedagainst the spring bias force to establish communication via the flowrestriction 26 between the pressure air source 25 and an aircommunication passage 34. Since the air shut-off valve 30 is in itsinactivated open position, there is free communication to the rear endof the cylinder 20, i.e. the driving side of the actuator piston 21. Atthe same time, however, the idling side of the piston 21, i.e. thepiston rod side, is prevented from being vented through conduit 35 inthat the shut-off valve 31 is closed. This is because the positionsensing valve 29 is activated by the piston 21 and supplies pressure airto the maneuver side of the shut-off valve 31. However, due to a largerpressurised area at the rear end of the piston than at the piston rodend, and due the vertical orientation of the actuator 14 and the totalweight of the piston 21, piston rod 22 and the working implement 17, acertain downward movement of the piston 21 will take place, long enoughto deactivate the valve 29 and stop pressurising the valve 31 to closedposition.

Now, the air shut-off valve 31 is shifted to its inactivated springmaintained open position to duct away vented air from the actuator 14through the communication passage 35 and the directional valve 24.Thereafter, the piston 21 is able to start moving downwards, to the leftin FIG. 2, so as to perform a crust breaking working stroke.

Due to the flow restriction 26 in the directional valve 24, the air feedto the actuator 14 takes place slowly, and since there is no flowrestriction in the vent passage of the valve 24, the air on the idlingside of the piston 21 will be vented to the atmosphere substantiallywithout any back pressure. The restricted air feed to the actuator 14prevents pressure from being built-up on the driving side of the piston21 to a higher level than what is actually needed for the piston 21 toperform a working stroke and to reach its fully extended position. Incase of a massive crust layer, a high pressure is required to move thepiston, and as long as the end position sensing valve 28 is notactivated, pressure air is continuously fed into the actuator cylinder20 successively increasing the pressure until the piston 21 eventuallyreaches its fully extended position and the end sensing valve 28 isactivated. When activated, the end sensing valve 28 opens upcommunication through the conduit 33 between the start signal conduit 23and the maneuver side of the shut-off valve 30 making the latter shiftto closed position. Thereby, the pressure air feed to the actuator 14 isstopped at once. An o.k. signal may be obtained via a conduit 37connected downstream of the end sensing valve 28. Such a signal may beused for remote control of the process.

The above described condition will prevail until the start commandsignal in conduit 23 is discontinued. The actuator piston 21 remains inits fully extended position, and no further pressure air is supplied tothe driving side of the piston 21.

When the start command signal in conduit 23 is discontinued, thedirectional valve 24 returns by spring force to its original position,to the left in FIG. 2, wherein instead the pressure air source 25 isconnected to the piston rod side of the actuator piston 21 via passage35. This communication is open since the end position sensing valve 29occupies its inactive closed position, and the air shut-off valve 31occupies its spring maintained open position. Venting of the rear idlingside of the piston 21 is established in that the pressure of the startcommand signal supplied via conduit 33 and the activated valve 28 stopsacting on the maneuver side of the shut-off valve 30 making the latterreturn to its inactive open position.

Now, the piston 21 starts moving upwards, to the right in FIG. 2, andbecause of the air feed restriction 27 in the directional valve 24, nomore pressure air is supplied to the actuator than what is needed tolift the piston 21, piston rod 22 and working implement 17 back to theirupper rest positions. The upper or right hand side of the piston 21 isvented through passage 34. As soon as the piston 21 reaches its fullyretracted position, the end sensing valve 29 is shifted to its openposition, against a spring bias force. Thereby, communication isestablished between the maneuver side of the shut-off valve 31 and thepressure air source 25 via a passage 38, resulting in a shifting of theshut-off valve 31 to its closed position, as illustrated in FIG. 2. Asin the opposite end position, an o.k. signal may be obtained via conduit39 connected downstream of the end position sensing valve 29.

From the above description of the actuator system it is apparent that bythe employment of the air shut-off valves 30,31 and the end positionsensing valves 28,29 there is obtained an instantaneous pressure airshut-off as the piston 21 reaches either one of its extreme endpositions. Whereas the directional valve 24 normally has to be locatedat a distance from the actuator 14 and the harsh environment in theclose vicinity of the electrolytic bath, the shut-off valves 28,29 whichare of a simple and rugged design may be located close to the actuator14 so as to accomplish a very quick and distinct air shut-off withoutany unnecessary delays. The combination of end position sensing valvesand separate air shut-off valves provides a substantially improvedpressure air economy, because the needed air pressure and the consumedair volume are continuously and automatically kept at a minimum level.

In FIG. 3, there is illustrated an alternative embodiment of theinvention, wherein air feed flow restrictions 26 a,27 a are integratedin the air shut-off valves 30 a,31 a. This means a further improvementof the actuator control function, because in this case the pressuredrops caused by the long conduits between the directional valve 24 andthe actuator 14 are minimized since a less sensitive full pressure airfeed is maintained all the way up to the shut-off valves 30 a,31 a. Inorder to avoid flow restrictions on the vented side of the actuatorpiston 21, the shut-off valves 30,31 have been provided with shunts40,41 including check valves 42,43.

By the location of the air feed restrictions 26 a,27 a to the shut-offvalves 30 a,31 a, it is made possible to obtain pressure air supply tothe position sensing valves 28,29 via conduits 33 a,38 a connected tothe conduits 34,35 where full pressure is available when required. So,air supply conduits 33 a and 38 a may be connected to the conduits 34,35at a location close to the actuator 14 instead of a location close tothe directional valve 24. This reduces the number of conduits betweenthe directional valve 24 and the actuator 14. It also means that thedirectional valve 24 can be located at a distance from the actuator 14away from the aggressive atmosphere around the electrolytic bath. Afurther advantage gained by this alternative location of the air feedrestrictions 26 a,27 a is a less complicated directional valve 24, i.e.the directional valve 24 may be of a simple conventional design.

A slight variation of the above described device is illustrated in FIG.4. Instead of having a spring biassed directional valve 24 whichautomatically returns to its operation start position as soon as thestart command signal is discontinued, there is employed a bi-stabledirectional valve 24 a. An OR-gate 36 is connected between the o.k.signal conduit 37 and one maneuver side of the directional valve 24 a.By this OR-gate 36 it is possible to reset the directional valve 24 aeither automatically by the o.k. signal obtained from the end positionsensing valve 28 or by a reset signal provided by a remote control unit(not shown).

It is to be noted that the embodiments of the invention are not limitedto the described examples but may be freely varied within the scope ofthe claims.

For instance, the actuator system according to the invention may be usedat alumina reduction pots where the crust layer breaking devicecomprises a horizontal crust breaking beam. In that application, oneactuator is connected at each end of the breaking beam for vertical,substantially parallel movement of the beam through the crust layer. Thetwo actuators are fed with pressure air by a common directional valve,and the flow restrictions in the feed passages of the directional valvewill be effective in distributing the air flow to both actuators inresponse to their individual instant load, such that the actuator havingthe lowest load gets the most pressure air. This means that the drivepressures in the actuators are automatically adapted to the actualindividual load level, such that when one of the actuators has reachedits extreme positions and the other has not the latter will becontinuously pressurised until it has reached its extreme end positionas well. Meanwhile, the air supply to the first actuator to reach itsextreme end position is cut off by the respective air shut-off valve.

What is claimed is:
 1. Pneumatic actuator system, comprising: one ormore piston-cylinder type actuators (14) each having a working piston(21) with a load engaging piston rod (22), a control circuit including adirectional valve (24;24 a) connected to a pressure air source (25) andarranged to direct pressure air to alternative driving sides of theworking piston (21) of each actuator (14) for accomplishing movement ofthe working piston (21) in alternative directions, characterized in thateach actuator (14) is provided with end position sensors (28,29) fordetecting and indicating the extreme end positions of the working piston(21), air feed shut-off valves (30,31; 30 a,31 a) connected to said endposition sensors (28,29) and arranged to cut off the air feed to thecurrent driving side of the working piston (21) as an extreme endposition is reached and indicated by the respective end position sensor(28,29), and air flow restrictions (26,27;26 a,27 a) arranged to limitautomatically the air feed flow to the current driving side of theworking piston (21), thereby limiting automatically the pressure airvolume supplied to the driving side of the working piston (21) at lowpiston rod load magnitudes.
 2. Actuator system according to claim 1,wherein said directional valve (24;24 a) is located remotely from theactuator or actuators (14), whereas said air feed shut-off valves(30,31; 30 a,31 a) form a unit together with the respective actuator(14).
 3. Actuator according to claim 2, wherein said air flowrestrictions (26 a,27 a) are located in said air feed shutoff valves (30a, 31 a).
 4. Actuator according to claim 2, wherein said shut-off valves(30,31; 30 a,31 a) are mounted on the outside of the respective actuator(14), whereas said end position sensors (28,29) are built-in in therespective actuator (14).
 5. Pneumatic actuator system for crustbreaking in electrolytic aluminum reduction baths (10), comprising oneor more piston-cylinder actuators (14) each having a working piston (21)with a piston rod (22) connected to a crust breaking implement (17), acontrol circuit including a directional valve (24;24 a), air flowrestrictions (26,27) inserted between the actuator (14) and thedirectional valve (24;24) for restricting the air feed flow to thecurrent driving side of the working piston (21), characterized in thateach actuator (14) is provided with end position sensors (23,29) fordetecting and indicating the extreme end positions of the working piston(21), air feed shut-off valves (30,31; 30 a,31 a) connected to said endposition sensors (28,29) and arranged to cut off the pressure feed tothe current driving side of the working piston (21) as an extreme endposition of the working piston (21) is reached and indicated by therespective end position sensor (23,29), and air flow restrictions(26,27) disposed between the actuator (14) and the directional valve(24;24) for restricting automatically the air feed flow to the currentdriving side of the working piston (21) at low piston rod loadmagnitudes, wherein said end position sensors (28,29) and said air feedshutoff valves (30,31; 30 a,31 a) are disposed integrally with theactuator (14) to form a working unit to be located at the electrolyticreduction bath (10), whereas said directional valve (24,24 a) is locatedremotely from the electrolytic bath (10).
 6. Actuator system accordingto claim 5, wherein said flow restrictions (26 a,27 a) are integratedwith the air feed shut-off valves (30 a,31 a).
 7. Actuator systemaccording to claim 6, wherein two actuators (14) have their workingpistons connected to a common crust breaking beam, said actuators (14)sharing a common remotely located directional valve (24;24 a) butcomprising separate end position sensors (28,29) and air feed shut-offvalves (30,31; 30 a, 31 a).
 8. Actuator system according to claim 6,wherein each actuator (14) operates a single-point crust breakingimplement (17) which extends in a substantial co-axial dispositionrelative to said piston rod (22).
 9. Actuator system according to claim5, wherein two actuators (14) have their working pistons connected to acommon crust breaking beam, said actuators (14) sharing a commonremotely located directional valve (24;24 a) but comprising separate endposition sensors (28,29) and air feed shut-off valves (30,31; 30 a, 31a).
 10. Actuator system according to claim 5, wherein each actuator (14)operates a single-point crust breaking implement (17) which extends in asubstantial co-axial disposition relative to said piston rod (22).